1. Field of the Invention
The present invention relates to a method of handover and route diversity operations in a mobile radio communication system such as an automobile telephone system.
2. Description of the Background Art
In a mobile radio communication system, the efficient utilization of limited frequency spectrum is one of the major technical problems. For this reason, a so called cellular system is adopted in general. In this cellular system, the efficient frequency spectrum utilization is achieved by providing a plurality of mutually distanced base stations within a service area of the system so as to enable a so called co-channel reuse which is a repetitive use of the same frequency at different base stations, where the base stations are sufficiently distanced from each other to avoid a co-channel interference. In such a cellular system, an area covered by each base station is called a cell.
In this cellular system, a higher frequency spectrum utilization efficiency can be achieved by making a radius of each cell smaller, because it becomes possible for the same frequency to be repetitively used at shorter distances. Such a use of smaller size cells is effective in increasing a system capacity regarding a number of subscribers that can be handled by the system.
However, as a consequence of an increased number of cells required for covering the entire service area, such a use of smaller size cells also necessitates an increased number of so called handover operations required to be performed when a mobile station moves from one cell to another during a single continuous communication. As a result, the capacities and sizes of each base station and a central control station controlling the base stations will be increased considerably and configurations of the base stations and the central control station will inevitably become quite complicated.
Namely, in the cellular system, as a mobile station which is communicating through one particular base station of one particular cell moves from that particular cell to a neighboring cell, the receiving level at that particular base station will gradually decreases, so that in order to continue this communication it is necessary to switch the base station through which the mobile station communicates from that particular base station of that particular cell to the base station of the neighboring cell. Here, in switching the base station, an active land transmission line connecting the central control station with the base station needs to be switched, while the frequency used for the communication by that mobile station needs to be switched from the traffic channel used in an original cell from which the mobile station is moving out to that of an idle traffic channel available in the base station of a destination cell to which the mobile station is moving in. Such an operation is known as the handover operation (which is sometimes also referred to as a handoff operation).
More specifically, as shown in FIG. 1, a conventional cellular system comprises a central control station 1, and a plurality (four in FIG. 1) of base stations 2 to 5 controlled by the central control station 1 through land transmission lines 11 to 14, respectively, where a mobile station 10 moves through a plurality (four in FIG. 1) of cells 6 to 9 associated with the base stations 2 to 5, respectively. In this cellular system, the central control station 1 is connected to an ordinary telephone network (not shown), such that a communication to and from the mobile station 10 can be transmitted through one of the base stations 2 to 5, the central control station 1, and the ordinary telephone network.
Now, assuming that the mobile station 10 is located in a cell 8 and communicating through the base station 4, as the mobile station 10 moves to the neighboring cell 7, the handover operation will proceed as follows.
(1) As a distance between the base station 4 and the mobile station 10 becomes greater than the cell radius, the receiving level at the base station 4 decreases below the specific level. When this decrease of the receiving level is detected by the base station 4, the base station 4 requests the handover of the communication of the mobile station 10 to the central control station I through the land transmission line 13.
(2) In response to this request for the handover from the base station 4, the central control station 1 commands the neighboring base stations 2, 3, and 5 to monitor the transmitted radio waves from the mobile station 10 through the land transmission lines 11, 12, and 14, respectively.
(3) In response to this command from the central control station 1, each of the base stations 2, 3, and 5 receives the transmitted radio waves of the traffic channel used for the communication of the mobile station 10 specified by the command from the central control station 1, measures the receiving level for this traffic channel, and reports the measured receiving level to the central control station 1.
(4) Then, the central control station 10 selects the base station for which the reported receiving level is the highest among the base stations 2, 3, and 5. In this exemplary case, the base station 3 will have the highest receiving level as the mobile station 10 is moving into the cell 7.
Next, the central control station 10 commands the selected base station 3 to report its idle traffic channel #N which is available for the handover of the communication of the mobile station 10.
(6) In response to this command from the central control station 1, the base station 3 reports the idle traffic channel #N to the central control station 1.
(7) Then, the central control station I commands the base station 4 to send a command signal for the handover to the traffic channel #N to the mobile station 10. Meanwhile, the central control station 1 also commands the base station 3 to activate the transmitter-receiver for the traffic channel #N, while switching the active land transmission line from that connected to the base station 4 to that connected to the base station 3.
(8) In response to the command from the central control station 1, the base station 4 sends the command signal for the handover to the mobile station 10.
(9) In response to the command signal from the base station 4, the mobile station 10 switches its traffic channel to the traffic channel #N specified by the command signal, so as to establish the communication through the base station 3.
Now, in addition to the increase of capacities and sizes of each base station and the central control station and the complication of configurations of the base stations and the central control station already mentioned above, such a conventional method of handover in the cellular system has a drawback that it is quite time consuming as it requires several signal exchanges between the central control station 1 and the base stations after the lowering of the receiving level is detected at the base station 4 in order to complete the handover operation.
As a consequence, in a case the cell radius is made smaller, it becomes possible for the mobile station to move further onto the next neighboring cell before the handover operation from one cell to its neighboring cell is completed, such that the highly undesirable failure of the handover of the communication could occur in the cellular system. To eliminate such a possibility for the failure of the handover, the cell radius cannot be made smaller than a certain limit value, and this limitation on the cell radius in turn creates an upper limit to the efficient frequency spectrum utilization.
Moreover, when the mobile stations with higher moving capability such automobile telephones and the mobile stations with lower moving capability such as portable telephones are involved in such a conventional cellular system together, the cell radius must be determined in accordance with the automobile telephones which can move faster, in order to prevent the occurrence of the failure of the handover, even though the cell radius so determined is not capable of achieving the highest frequency spectrum utilization efficiency for the portable telephones. Consequently, the base station such as that located in a heavily populated area where a number of portable telephone subscribers is large needs to have a large number of traffic channels assigned, but this in turn lowers the frequency spectrum utilization efficiency further.
On the other hand, when the cell radius is determined in accordance with the portable telephones, it becomes difficult for the system to handle the frequent handover operations required by the communication of the portable telephone used on a fast moving object such as an automobile.
In addition, as the cell radius becomes smaller, an area between neighboring cells which is not well served by any cell may appear, and also the quality of service will be lowered drastically as soon as the mobile station moves out of the area covered by the cells, such that there is a great possibility for the subscribed portable telephone user to suffer from the poor quality of service.
Now, in a mobile radio communication, the radio transmission characteristic is quite adverse because of the large receiving level fluctuation and other causes, such that the occurrence of errors in the transmission signals is unavoidable. For this reason, the mobile radio communication system utilizes various error correction mechanisms. However, these error correction mechanisms are effective only with respect to the instantaneous fluctuation of the receiving level, and not very effective with respect to the relatively gradual variation of the receiving level caused by the shadowing of the transmission path due to the terrain and environmental features.
To such a relatively gradual variation of the receiving level, a so called route diversity reception in which the transmitted radio waves received by a plurality of base stations are utilized is known to be effective. For instance, the automobile telephone of NTT (Nippon Telegraph and Telephone) Corporation utilizes the base station route diversity reception for the control channel.
More specifically, a conventional route diversity reception is achieved as follows.
Namely, a conventional mobile radio communication system utilizing the route diversity reception has a configuration shown in FIG. 2, where the system includes a central control station 21 and a plurality of base stations 22 to 28 each of which is connected with the central control station 21 through land transmission lines 29 to 35, respectively.
In this mobile radio communication system, a signal transmitted from each of the base stations 22 to 28 to the central control station 21 has a format shown in FIG. 3 which includes a transmission signal 36 from a mobile station and an encoded receiving level information 37 indicating the receiving level at each base station. In other words, each base station relays the transmission signal 36 from the mobile station to the central control station 21 by attaching the receiving level information 37 to the transmission signal 36.
The central control station 21 then selects the base station for which the receiving level is the highest among all the base stations 22 to 28, and uses the transmission signal 36 transmitted from this selected base station as the transmission signal 36 received from the mobile station.
Such a conventional mobile radio communication system has drawbacks that the expensive land transmission line must be provided between the central control station 21 and each one of a plurality of the base stations and that the control function of the central control station 21 inevitably becomes complicated as a consequence of utilizing the route diversity reception.